Simple Persistence

--[[
Fixes PS#DE7E9F99
Out-of-sandbox code/data files could be overwritten using this due to environment weirdness.
Now persistence data is in its own folder so this will not happen.
]]
local persist_dir = "persistence_data"

local function load_data(name)
    local file = fs.combine(persist_dir, name)
    if not fs.exists(file) then error "does not exist" end
    local f = fs.open(file, "r")
    local x = f.readAll()
    f.close()
    return textutils.unserialise(x)
end

local function save_data(name, value)
    if not fs.isDir(persist_dir) then fs.delete(persist_dir) fs.makeDir(persist_dir) end
    local f = fs.open(fs.combine(persist_dir, name), "w")
    f.write(textutils.serialise(value))
    f.close()
end

return function(name)
    if type(name) ~= "string" then error "Name of persistence volume must be a string." end
    local ok, data = pcall(load_data, name)
    if not ok or not data then data = {} end
    local mt = {}
    setmetatable(data, mt)

    mt.__index = {
        save = function()
            save_data(name, data)
        end,
        reload = function()
            -- swap table in place to keep references valid
            for k in pairs(data) do data[k] = nil end
            for k, v in pairs(load_data(name)) do
                data[k] = v
            end
        end
    }

    function mt.__tostring(_)
        return ("[PersistenceStore %s]"):format(name)
    end

    return data
end